If any of the error conditions occurs, the function generates an invalid parameter error, as described in Parameter Validation. If the error is handled, the function returns EINVAL and sets errno to EINVAL.

The _mbsnbcat_s function appends to dest, at most, the first count bytes of src. If the byte that immediately precedes the null character in dest is a lead byte, it is overwritten by the initial byte of src. Otherwise, the initial byte of src overwrites the terminating null character of dest. If a null byte appears in src before count bytes are appended, _mbsnbcat_s appends all bytes from src, up to the null character. If count is greater than the length of src, the length of src is used in place of count. The resulting string is terminated by a null character. If copying takes place between strings that overlap, the behavior is undefined.

The output value is affected by the setting of the LC_CTYPE category setting of the locale; see setlocale, _wsetlocale for more information. The versions of these functions are identical, except that the ones that don't have the _l suffix use the current locale and the ones that do have the _l suffix instead use the locale parameter that's passed in. For more information, see Locale.

In C++, the use of these functions is simplified by template overloads; the overloads can infer buffer length automatically and thereby eliminate the need to specify a size argument, and they can automatically use their newer, more secure functions to replace older, less-secure functions. For more information, see Secure Template Overloads.

The debug versions of these functions first fill the buffer with 0xFD. To disable this behavior, use _CrtSetDebugFillThreshold.